1. Field of the Invention
The present invention relates to electric control cables, or power cables, used for conveying currents.
2. Description of Related Art
Such cables are used in various fields in industry, such as for example the automotive industry, where they are assembled into bundles for feeding electricity to various pieces of equipment. It is therefore necessary, in particular, for such cables to be as light in weight as possible, and to be compact, while nevertheless conserving good mechanical strength.
Such cables are conventionally made up of a plurality of strands of copper, generally twisted to form a twisted strand so as to increase the flexibility of the cable, and surround by an insulating sheath, e.g. obtained by extrusion.
The advantages of a cable with the above structure lie essentially in the simplicity of the fabrication method, and also in the fact that it can be crimped reliably to connectors. It suffices to strip the cable locally by removing a portion of the insulating sheath 30 where it is desired to place the connector, and then to mechanically compress a connector bushing around the stripped section of cable. In addition, copper intrinsically presents good mechanical traction strength.
In contrast, it has been found that the above cable makes use of a quantity of copper that is excessive compared with the real requirements corresponding to the quantity of electric current that is to be transmitted by the cable. More precisely, about half of the copper in the above cable structure is used for increasing the traction strength of the cable, and also for guaranteeing effective crimping.
Unfortunately, copper is becoming ever more expensive, and it is important to find new cable structures that reduce the quantity of copper used to as little as possible.
Various solutions are already known for composite cables in which copper strands are combined with a core of non-conductive material. In particular, U.S. Pat. No. 7,145,082 describes a control cable in which a plurality of conductor wires, e.g. copper wires, are twisted around a central core made up of a multifilament polymer of the aramid fiber type.
That type of cable makes it possible to reduce the quantity of copper used significantly, down to the value actually required for proper transmission of the signal, while conserving very good traction strength because of the use of aramid.
In contrast, although aramid possesses very high traction strength compatible with the values required, that type of material presents little elongation at break, typically of the order of only 3%. Those characteristics are shown by traction curve 1 in FIG. 2 plotting the traction force required as a function of elongation for aramid.
Throughout the utilization of lifetime of a cable, and in particular at the time it is being installed, e.g. inside a motor vehicle, or in the event of subsequent action on the cable for replacement or repair purposes, it can become necessary, or inevitable, that traction is applied on the cable in order to lengthen it. This applies in particular when it is desired to connect the end of the cable to a connection box that is situated in a location that it is difficult to access within the motor vehicle. If a cable having an aramid fiber core is pulled, whether intentionally or not, it does not lengthen.